The sum of all external factors, both biotic (living) and abiotic (nonliving), to which an organism is exposed. All of the outside factors to which an organism is exposed comprise its environment (Fig. 1). The environment has a tremendous influence on an organism's ability to live. Biotic factors include influences by members of the same and other species on the development and survival of the individual. Primary abiotic factors are light, temperature, water, atmospheric gases, and ionizing radiation, influencing the form and function of the individual. In general, for each environmental factor, an organism has a tolerance range in which it is able to survive. The intercept of these ranges constitutes the ecological niche of the organism. Different individuals or species have different tolerance ranges for particular environmental factors; this variation represents the adaptation of the organism to its environment. The ability of an organism to modify its tolerance of certain environmental factors in response to a change in them represents the plasticity of that organism. Alterations in environmental tolerance are termed acclimation. Exposure to environmental conditions at the limit of an individual's tolerance range represents environmental stress. See also: Adaptation (biology); Ecology; Physiological ecology (animal); Physiological ecology (plant)

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